This application's long term objectives are to understand the determinative factors that bring about development of the olfactory system, including: the role of odorant stimuli in defining the functional characteristics of nascent olfactory sensory neurons; the role of ingrowing afferent nerve terminals in organizing the anatomy of the olfactory midbrain; the possible involvement of glial cells in the development of olfactory lobe glomeruli; the factors controlling the growth and number of glomeruli as the animal ages; the path finding mechanisms important in assuring that in-growing sensory axons will arrive at and make synaptic contact with functionally appropriate central targets. In mammalian systems it is clear from behavioral findings that the developing olfactory system is characterized by much functional plasticity. Specifically, it has been demonstrated that environmental odorants may play a crucial role in the development of its functional organization. In salmonid fish, the prevalence of specific environmental odorant stimuli during the developmental smolt is known to condition these animals to their natal stream and is important in directing homing behavior in the later stages of their lives. Knowledge about how these influences modify the functional organization of the developing olfactory system will provide insights that can be translated into preventative prenatal care in humans. In the present proposal, our specific aims are l) to provide detailed characteristics of the growth of the olfactory midbrain in the presence and in the absence of the peripheral sensilla to determine how the appearance and growth of new olfactory lobe glomeruli are controlled by the supply of afferent fibers, and 2) to examine the extent of competition between existing and nascent afferent input, using heteromorphic antennule as an extraordinary source of afferent fibers to determine their affect upon the central synaptic connections made afferents from the normal ipsilateral antennule. Results from experimental manipulations will be analyzed using correlated light and electron microscopic observations. Competitive interactions will also be analyzed using electrophysiological techniques.